The increasing requirements for communication systems capacity can be satisfied either by accessing higher frequencies or by using the introduced frequencies more effectively.
The capacity of a radio communication system with limited bandwidth can in theory be doubled by using orthogonally polarized waves. Such a frequency reuse system transmits two broadband channels either by two orthogonal linearly polarized waves or by two opposite-sensed circularly polarized waves. The degree of isolation between the two channels is directly related to the polarization purity of the signal waves. Reduction of polarization purity (cross-polarization coupling) is caused in practice by non-ideal rf-components, by misalignment of the antennas, by rain and -- in the case of dual linearly polarized satellite communication systems -- by Faraday rotation. Some of these effects are time varying in nature.
In an operational system some kind of compensation network must be provided in order to minimize cross-polarization coupling of the two broadband channels.
The present invention relates to a control system for re-establishing the orthogonality of two signals, transmitted with orthogonal polarizations, which are each provided with a beacon signal and are cross-coupled in the relay link, the cross-coupled signals being reconstituted by means of a decoupling network which comprise at least two controllable members.
The principal considerations as to how such a cross-coupling can be eliminated have been discussed by T. S. Chu: Restoring the Orthogonality of Two Polarizations in Radio Communication Systems, Part I, BELL SYSTEMS TECHNICAL JOURNAL, Vo. 50, No. 9, Nov. 1971, pages 3063-3069, Part II, BELL SYSTEM TECHNICAL JOURNAL, Vol. 52, No. 3, March 1973, pages 319-327.
The compensation device proposed by Chu may be placed before or behind the propagation path. Due to cross-polarization, the signal waves are in general elliptical and nonorthogonal. The ellipses are fat when the transmitted signals are dual circularly polarized and they are slender in the case of dual linear polarization. The compensation suggested by Chu is performed in two steps. First the polarizations are made linear -- but not in general orthogonal -- by means of a coordinate rotation and differential phase shift. Second the polarizations are orthogonalized by a further, independent coordinate rotation combined with a differential attenuation. Hence, four parameters must in general be adjusted to compensate for a given depolarization condition.
Because of the time dependancy of the cross-polarization effects the compensation network must be incorporated in an automatic control system.